This invention relates to a web tension control system. It relates more particularly to a system of this type which can maintain a web festoon dancer at a selected reference position over a wide range of operating conditions.
Tension control systems are used in conjunction with splicers, unwinders, rewinders and the like to maintain substantially uniform tension in the web. In the case of an unwinder, web drawn from a roll passes through a web storage festoon which normally stores a selected quantity of web and thence to a web-consuming machine such as a printing press that draws web at a constant speed from the festoon. The festoon includes a preloaded dancer that imparts a selected tension to the web drawn from the roll, and that tension is maintained substantially constant by using the dancer position to control the braking torque applied to the unwinding web roll.
If the web tension becomes higher than the desired value, the dancer is moved in one direction from its reference position, with the result that braking torque applied to the roll decreases. Thereupon the roll unwinds at a faster rate, thereby decreasing the tension in the web so that the dancer tends to return to its reference position. On the other hand, if web tension falls below the desired value, the dancer moves in the opposite direction so that the braking torque applied to the roll is caused to increase. Accordingly, the tension in the web is increased and the dancer is returned to its reference position.
A rewinder operates in more or less the same way except that the festoon is located upstream from the roll of web being rewound and the roll is positively driven by a variable torque motor in accordance with dancer position. Since the two applications are similar, we will describe the present system only in conjunction with an unwinder.
Typically, the festoon dancer position is monitored by coupling the dancer to a potentiometer that varies a voltage in accordance with the dancer position. That voltage is compared with a fixed voltage corresponding to a selected dancer reference position to develop a dancer position error signal. The error signal is then applied to a low gain position amplifier whose output controls a transducer. That element, in turn, varies the pressure applied to a hydraulic brake associated with the unwinding web roll as needed to maintain the dancer at the selected reference position.
Conventional systems also vary braking torque in accordance with the velocity of the festoon dancer toward or away from its reference position. In addition, some prior systems vary the gain of the control system according to the size of the web roll. The instantaneous radius of the roll is monitored by a follower arm, photo-detector, or, through the use of tachometers to compare the speed of the unwinding roll with that of a fixed diameter guide roller. An example of a prior system with all of these features is shown in U.S. Pat. No. 3,822,838.
In order to provide a consistent amount of festoon storage at the moment a splice cycle is initiated, the tension control system should be arranged to position the festoon dancer at a reference position representing a selected percentage of the maximum storage capacity of the festoon. That percentage should be quite high so that the festoon assuredly stores enough web to satisfy the needs of the downstream web-consuming machine as the expiring web roll is braked to a stop, spliced to the leading end of the new roll and the new roll accelerated up to line speed. Yet the percentage should not be so high that the festoon cannot accommodate normal web tension upsets encountered during the splice sequence that cause the dancer to move further toward its maximum storage position. Desirably, the festoon dancer is referenced to a position corresponding to 80% of its maximum storage position to assure an adequate web supply during splicing, particularly at high web speed. Moreover, that position should be maintained to an accuracy of the order of .+-.5% for all steady state operating conditions of the system.
It has been found that in order to maintain the dancer at its reference position with the desired accuracy, the tension control system must have a relatively high gain. However, it has been further found that high gain systems are quite difficult to stabilize, especially if they rely on slow response devices to brake the web roll such as the pneumatic transducers and hydraulic brakes that are prevalent in the industry.
Further, conventional web tension control systems have not been able to operate reliably to provide the wide range of braking torques required for different tension levels encountered in different web handling applications.